by Peter Török
New density measurements and Calibration
As advised from the last meeting we placed the measuring jug on an electronic scale, reset the scale to zero and filled it with 400g worth of water. We found that there was a small error in the jug which produced the systematic errors in our previous measurements. We marked the real 400ml mark with a pen and took new volume and weight measurements. I also skinned two of the potatoes to check the removal of skin didn’t significantly change the density (The rows highlighted in the same colour correspond to the same potato). Below is a table of results of potatoes with the skin on:
The starch concentrations were worked out using the published EU starch-density data.
In general the graph shows that heavier potatoes are denser and contain more starch. This agrees with the data taken before and the new density measurements fall within the upper and lower density limits set by the EU data.
Starch Content Pre and Post blanching
The next experiment I did aimed to quantify the starch release near the surface of the potato during frying. The motivation behind this was to see if there was an increased release of starch near the surface with blanching time. It is thought that this build up and release of starch near the surface is what contributes to increased crust thickness with longer blanching times.
In the first starch testing experiment, I blanched strips for different times and cut approximately a thin strip of approximately 1mm thickness near the surface. I then homogenized the sample using a pestle and mortar and added it to 80ml of a tincture of iodine. The tincture was prepared by mixing 0.5ml of 8.6% concentrated iodine solution (by mass) to 500ml water. What I found was that all the samples turned the same colour regardless of blanching time. This is due to the fact that the act of homogenizing the strips releases starch physically by bursting the cell walls so this does overrides any starch release from blanching.
Also from the previous starch experiment where I stained the surface of whole chips using iodine solution all the blanched strips turned the same colour. In light of all of this I decided the best way to quantify the surface starch concentration/release was to cut a thin strip, immerse it in iodine solution, stir it for a fixed amount of time using a spoon and then to photograph the sample.
Main Starch Experiments and Colour Analysis
The procedure for the following colour tests were to place a thin 1mm strip of potato (for range of blanching times) into an 80ml tincture of iodine (prepared in exactly the same as before). The samples were then photographed using a canon 550D DSLR from a distance of 45cm using a 50mm lens. The samples were taken against a white background and the lighting conditions were kept constant. The images are analysed using GNU Image Manipulation Programme by averaging the intensity of the RGB spectrum at various points down the centre line of each beaker (shown below):
In the 24 bit RGB representation, the intensity of red, green and blue are given as integers between 0 and 255 (where 0,0,0 is black and 255,255,255 is white). I decided to invert the scale as strongest iodine-starch reactions produced the darkest blue-purple colours. Then I normalised the intensity values against a control sample which was created by mixing 0.25g of potato starch (assumed to be 100% pure) and 250ml of iodine tincture. I also diluted the control sample by 1:4 and 1:16 of the original concentration. 10ml of each potato starch mixture was then added to 70ml of iodine to produce 3 beakers of known relative starch concentrations:
(From left to right 1, 1:4, 1:16 concentration of control sample detailed above, note all RGB values are normalised against the RGB intensities of the left beaker)
The graph above shows the relative RGB intensities relative to the darkest control sample. We ran out of time last session – my friend had to leave early, but I intend to take more values to build up a comprehensive spectrum for starch concentration vs RGB intensity. The Red and Green values overlap so only the Red value was shown (as Red contributes more to the blue-purple colour of iodine reaction).
The graph below shows the Normalised RGB Intensity against blanching time of potato strips shown before (1mm slice thickness):
The graph shows that there is a significant release of starch near the surface of the potato strip during 8 – 11 minutes. From sensory analysis we believe that longer blanching times are linked to the crispiest and crunchiest fries (To be confirmed from mechanical testing). This is approximately the point where the potato starts to weaken and break down.
Whole potato strip tests
In addition to testing thin 1mm strips of potato near the surface I also tried placing whole 9x9mm potato strips into the same iodine solutions at varying blanching times/minutes:
These were inconclusive as the colour remained very similar for the first 4 samples. The plan for Monday is to create a comprehensive control curve by adding more data to the current curve. Then it will be possible to determine relative starch release as a function of blanching time.